1. Field of the Invention
The present invention relates to a radiation sensing apparatus to be used in radiation medical equipment and the like, and to a sensing method of the radiation sensing apparatus. For example, the present invention relates to a radiation sensing apparatus for performing an imaging diagnosis and the like using a high energy radiation such as an X-ray and a gamma ray, and to a sensing method of the radiation sensing apparatus.
2. Related Background Art
A method for obtaining a transmitted image of a subject by the use of a radiation having strong permeability such as an X-ray is generally used widely in a non-destructive examination for an industrial use and in a field of a medical diagnosis. Digitization has been advancing in various fields of medical treatments. In the field of the X-ray diagnosis, a two-dimensional digital X-ray highly sensitive transmission apparatus for sensing a visible light image with a TV camera after converting an input radiation into visible rays with a scintillator (fluorescent substance) and an image intensifier (I.I.) has been developed.
Furthermore, a radiation sensing apparatus using a charge coupled device (CCD) type image sensor or an amorphous silicon two-dimensional image sensor as sensing means in the place of the image intensifier (I.I.) to be combined with a fluorescent substance has recently been disclosed in U.S. Pat. No. 5,315,101, U.S. Pat. No. 5,693,948 and the like.
The following advantages can be cited as the advantages of the digital sensing apparatus to analog photography. For example, sensing without use of any film can be attained. The quantity of obtainable information can be increased by image processing. It is possible to make a database. Moreover, it is useful in a scene of an urgent medical treatment to be able to display a sensed image instantly at the scene.
Moreover, Japanese Laid-Open Patent Application No. 11-99144 discloses a flat panel type sensing apparatus applied to a movable X-ray sensing apparatus which can be moved to a bed side to perform X-ray sensing of a patient who has a difficulty of walking due to a serious injury or the like. Because the flat panel type sensing apparatus can easily be made to be small in size, it has also been considered to apply the flat panel type sensing apparatus to a portable type X-ray sensing apparatus for performing an urgent X-ray sensing at a scene of an accident.
Moreover, Japanese Laid-Open Patent Application No. 09-107503 discloses to apply a CCD type image sensor to an X-ray sensing apparatus for dental use or for mammography, in which a start of X-ray irradiation on the basis of an output of the image sensor can be detected without connecting the image sensor with an X-ray irradiation apparatus directly.
An example of a sensing sequence in case of detecting a start of X-ray irradiation on the basis of an output of an image sensor without connecting the image sensor with an X-ray irradiation apparatus directly will be explained by reference to FIGS. 10A to 10E. FIG. 10A shows an output signal of an exposure switch. FIG. 10B shows a state of irradiation of an X-ray. FIG. 10C shows a driving state of a CCD type image sensor. FIG. 10D shows reset signals. FIG. 10E shows a signal output.
The output signal of the exposure switch is a two-stage signal. A first stage of the signal is a sensing request signal. A second sage of the signal is an exposure signal. The CCD type image sensor starts its operation by a photographer""s action of turning on a drive switch (not shown) separated from the X-ray irradiation apparatus. The photographer turns on the first stage exposure switch, and subsequently turns on the second stage exposure switch.
The CCD type image sensor performs its reset operation repeatedly until the image sensor detects X-ray irradiation, thereby waiting exposure while reading out a dark current periodically. In case of the CCD type image sensor, because accumulated electric charges are transferred to the outside to be read out at every read-out operation, only destructive read-out can be performed due to its configuration. The CCD type image sensor and the like have the characteristic of accumulating a dark current even in the state of no signal. Because the dark current is one of the causes of noises at the time of sensing a minute signal, a reset operation for decreasing the charges due to the dark current accumulated in the image sensor is required just before sensing for obtaining a useful subject signal. Naturally, the read-out accompanied by the reset operation is a destructive read-out. When X-ray irradiation is detected by the destructive read-out in a frame operation A2 shown in FIG. 10C, signal accumulation due to the X-ray irradiation is started at a time of the completion of the reset operation of the frame.
However, the following two points become problems in the drive technique described above.
One of them is that useless exposure to an X-ray occurs. Because the signal in the frame in which the X-ray is detected is read out by the destructive read-out, the signal is not used for an image signal at all. In the worst case, the X-ray irradiation for the period of time of read-out of one frame becomes useless. Because an exposure dose should be suppressed to the utmost in an radiation sensing for a diagnosis having a medical object, it is desired to remove such uselessness.
Moreover, in such a destructive read-out, the end of the X-ray irradiation cannot be detected. Then, the read-out is performed by terminating signal accumulation after a predetermined period of time set before sensing in advance by anticipating a time when the X-ray irradiation is ended. A dark current in the period of time from the termination of the X-ray irradiation to the read-out is a cause of noises, and causes a deterioration of the image quality of an obtained image.
Accordingly, in case of the sensing regarding the image quality and the exposure dose as important, a radiation irradiation apparatus and a radiation sensing apparatus are electrically connected with each other to synchronize the timing of an X-ray exposure between them by exchanging signals between them. To put it concretely, a reset operation (initialization operation) is started at a sensing request signal, and the reset operation is repeated while the radiation sensing apparatus is waiting an exposure signal. After the reset operation performed at the time of the recognition of the exposure signal is terminated, an exposure permission signal is transmitted to a generation apparatus. Consequently, the exposure of the radiation exposure apparatus, and a photographer as well, is limited in accordance with the reset timing.
An example of a sensing sequence of the above-described radiation sensing apparatus is described by reference to FIGS. 11A to 11E. FIG. 11A shows an output signal of an exposure switch; FIG. 11B shows the state of irradiation of an X-ray; FIG. 11C shows a driving state of a CCD type image sensor; FIG. 11D shows reset signals; and FIG. 11E shows a signal output.
The output signal of the exposure switch is a two-stage signal. A first stage of the signal is a sensing request signal. A second sage of the signal is an exposure signal. The radiation sensing apparatus starts its reset operation by recognizing the first stage output signal of the exposure switch. After that, the radiation sensing apparatus performs the reset operation repeatedly until the apparatus detects the second stage signal of the exposure switch to wait exposure while reading out a dark current periodically. When the radiation sensing apparatus detects the second stage signal of the exposure switch, the radiation sensing apparatus allows the radiation irradiation apparatus to expose at a time of the completion of the initialization at that time, and the radiation sensing apparatus starts signal accumulation.
However, the drive technique described above have the problems of the following two points.
One of them is the occurrence of the delay of real sensing to the exposure signal. Because the exposure should be allowed after the completion of the reset operation, with respect to the input timing of the exposure signal which the photographer expects, the exposure delays for the period of time of reading out one frame in the worst case. On the other hand, in the radiation sensing for a diagnosis having a medical object, it is desired to perform exposure and signal accumulation while escaping the breezing of a subject, the shaking of the body and the like. Consequently, it becomes important to perform radiation sensing at the instance when the photographer observing and controlling the subject wants to sense the subject. The above-mentioned delay of sensing to the exposure signal brings about to miss the optimum instance and to decrease the information of the subject.
The other one of them is a problem caused by the exchanges of signals between the apparatus for generating a radiation and the sensing apparatus. That is, a radiation irradiation apparatus using an X-ray film including a silver salt or the like is obliged to be remodeled for the signal exchanges, and it becomes necessary due to the installation of the image sensor to calibrate exposure timing to each radiation irradiation apparatus severally. This makes the installation very troublesome. In particular, there are many urgent cases which require the use of movable X-ray sensing apparatus or a portable X-ray sensing apparatus, and therefore it is required to eliminate such troublesomeness.
Consequently, it is desirable to make the signal connection between the radiation irradiation apparatus and the radiation sensing apparatus unnecessary, and to permit a photographer to expose at arbitrary sensing timing, and further to make it possible to obtain a signal at a high speed response.
An object of the present invention is to provide a radiation sensing apparatus which does not need signal connection between a radiation sensing unit and a radiation irradiating unit, and a sensing method of the radiation sensing apparatus.
A radiation sensing apparatus of an embodiment of the present invention for achieving the above-mentioned object comprises a sensing unit arranged to sense a subject image to be obtained by irradiating a radiation from a radiation source to a subject, the sensing unit being capable of reading out non-destructively, and a detection circuit arranged to detect a start and/or a stop of irradiation of the radiation on a basis of a signal obtained from the sensing unit by the non-destructive read-out.
Moreover, another embodiment of the present invention provides a sensing method of a radiation sensing apparatus including a sensing unit arranged to sense a subject image to be obtained by irradiating a subject with a radiation from a radiation source, the sensing unit being capable of reading out non-destructively, the method comprising a step of detecting a start and/or a stop of irradiation of the radiation on a basis of a signal obtained by the non-destructive read-out.
The other objects and features of the present invention will be clear in the following descriptions in the specification and the attached drawings.